Change offer tests to use oscar_testsupport

tests/unit/offer/shipping_percentage_benefit_tests.py

@@ -5,7 +5,7 @@ from django_dynamic_fixture import G
 
 from oscar.apps.offer import models
 from oscar.apps.basket.models import Basket
-from oscar.test.helpers import create_product
+from oscar_testsupport.factories import create_product
 
 
 class TestAPercentageDiscountAppliedWithCountCondition(TestCase):